The invention concerns briquettes for the liquefaction of metallurgical slag, composed of a mixture consisting of slag from secondary aluminium processing as the basic ingredient, plus a polymeric bonding agent and water. The invention also concerns a method for production of such briquettes.
During the manufacture of aluminium, various kinds of slag are created containing different quantities of accompanying elements and compounds. At the present time, slag from secondary aluminium processing constitutes a waste product in the manufacture of aluminium and occupies a large amount of utility space in the form of heaps. In addition, when this slag is stored in heaps, it decomposes under the influence of atmospheric conditions and releases foul-smelling and harmful ammonia, causing significant deterioration of the environment. During the process of dumping, this slag gradually acquires a fine grainy consistency, thus generating a quantity of mineral and aluminate dust.
Slag from aluminium production is further processed by crushing and washing out, so that the residue aluminium may be obtained. The remaining part, which consists of a mixture of oxides, chlorides, fluorides and metal particles, particularly on a base of Al, Na, K and Si, is mostly dumped out or is very difficult to use.
Much of the technology that has already been introduced is applied to the use of the metallic parts of scum, or these scums and slags are washed out with water after granulometric treatment by crushing and milling. The first method generates a segment of waste that cannot be processed, i.e., the already mentioned mixture of metal particles and oxides.
In other cases where these waste products are extractible etc. after milling, a large quantity of gases are produced, including ammonia, acetylene, methane, sometimes arsines, phosphines, in addition to salt solutions and sludge. A drawback of these manufacturing processes is the creation of dangerous gases, salt solutions and sludge, whose liquidation requires large quantities of energy or complicated apparatus.
For example, in the Czech published patent application PV 830-95, a method is described for processing the dust fractions of aluminium scum; according to this method, the scum and aluminium waste are leached, without any kind of treatment, and dissolved in an air-excluding environment in a leaching solution containing chloride ions; in this environment, the carbides and nitrides decompose and the aluminium reacts. The resultant ammoniac, hydrogen, methane, appropriately the phosphines, arsines and other products are burned at a temperature of 800-1,000xc2x0 C. The remaining pulp is filtered off. A mixture of chloride salts is crystallised out from the water solution under heat. The filter cake is dried and without the presence of carbides, carbonitrides, metal aluminium etc. is granulated into pellets or briquettes.
The disadvantage of this method is the complicated machinery, manufactured mainly from rust-free material or material otherwise protected against corrosion, with problems in the capture of the released gases for hygienic and environmental reasons. This relatively complicated method is demanding in terms of energy consumption and is relatively costly in terms of investment and manufacture.
A certain improvement is seen in the complex agent described in the Czech published patent application PV 2914-90, which is designed for the liquefaction of slag with desulphurising effects. Here, in the fining process of steel-making, cryolite deposits created as waste substances during the electrolytic manufacture of primary aluminium are used as diluting agents for slag. The complex agent may be used in crushed state without further treatment or the addition of other components. The invention does not solve the problem of using the waste from the manufacture of secondary aluminium, in which it is usually necessary to secure the consolidation of the powder and fine-grain components of the slag from this manufacturing process.
In metallurgy, during the melting of iron and steel to reduce viscosity and facilitate the removal of slag during melting, fluorite, a low-profile and relatively expensive substance, is used as one of the components for liquefying the slag.
Slag from secondary aluminium processing, which up to now has been treated as manufactural waste, could be a valuable substitute for fluorite. Slag from secondary aluminium processing cannot be used in ferrous metallurgy as a liquefying agent in its original loose or powder form, as handling is inconvenient and makes the work-place very dusty in a manner contrary to industrial safety regulations.
Therefore, when slag from secondary aluminium processing is used in ferrous and steel metallurgy, it is suitable to process it into briquette form, enabling the ecological improvement of the environment and the work-place.
The manufacture of briquettes from slag makes use of various types of bonding agents, such as water glass, lime milk, molasses and other types of suitable adhesive substances. These substances, however, do not ensure a sufficient degree of strength in the briquettes.
Austrian patent AT 390 965 describes technology for the processing of aluminium scum, i.e. coarse-grain dross which are crushed. Metal aluminium is thus obtained, and the solid residue is further It processed into briquettes for the liquefaction of metallurgical slag. As the bonding agent, complex polymers with large amounts of radicals are used, such as the methyl group CH3, the ammonium group NH4, the phenyl groups, the cyanide groups CN and metal traces. These polymers are manufactured by means of complicated technology. In addition, at high temperatures they decompose and release poisonous substances such as phenol and hydrogen cyanide. Very high temperatures, ranging from 1,800 to 2,800xc2x0 F. (900-1,540xc2x0 C.) are used to polymerise the mixtures. Heating to such temperatures makes the technical process of polymerisation expensive and complicated. The granulation technology mentioned above is not suitable for slag from secondary aluminium processing, as its large reactive capacity causes a decomposing reaction to take place in the granules, which in turn causes the granules to lose their strength and resistance to decomposition. The highest strength value for the dry granules thus obtained is 21.9 psi, corresponding to 0.16 MPa, which is not sufficient for briquettes from aluminium slag. The highest strength value is obtained by the addition of clay to the mixture. But the addition of clay for the purpose of binding the particles of aluminium slag also seems to be unsuitable, owing to the considerable increase in the quantity of SiO2 in the mixture and to the high firing temperature of 2,800xc2x0 F. (1,540xc2x0 C). Reducing the quantity of clay leads to rapid reduction in the strength of the briquettes down to 0.016-0.02 MPa. For these reasons it is clear that the composition of bonding agent and manufacture of granules is not very suitable for the processing of slag from secondary aluminium processing into briquettes.
The purpose of this technical discovery is to obtain a cheap liquefying agent from slag from secondary aluminium processing that has identical properties to fluorite in the liquefaction of metallurgical slag, in such a manner or with such equipment as to minimise the disadvantages outlined above.
This aim will be achieved with briquettes for the liquefaction of metallurgical slag composed of a mixture of slag from secondary aluminium processing, as the basic component, a polymeric bonding agent and water, in accordance with this invention. The substance of this invention lies in the fact that the mixture contains, by weight, 85-98% by weight slag from secondary aluminium processing, 1-8% by weight polyvinyl acetate dispersion as the polymeric bonding agent, and the remainder water; and that the briquette contains water as residue humidity at a quantity of up to 1.5% by weight, ideally to 0.5% by weight, as related to the total weight of the briquette, where the briquette has a specific weight ranging from 1,500-5,000 kg.mxe2x88x923 and a strength under pressure of at least 20 MPa.
The ideal content of the mixture is, by weight 90-92% slag from secondary aluminium processing, 1-3% polyvinyl acetate dispersion and the remainder water.
It is also beneficial when the mixture also contains 0.5-2% by weight unsaturated acid selected from the group that includes oleic acid and/or palmitic acid and/or stearic acid as a hydrophobic ingredient.
In its advantageous version, the briquette has a cylindrical shape.
The briquette according to this invention is obtained by the method of manufacture according to this invention, the essence of which is that the various components of the mixture, i.e. slag from secondary aluminium processing, polyvinyl acetate dispersion as bonding agent, water, with the possible addition of a hydrophobic ingredient, are blended into a homogenous state in a mixer in a time of 10-60 seconds. The homogenous mixture thus obtained is then pressed at a specific press power of 110 to 130 MPa. The briquette is then dried at a temperature of 120 to 200xc2x0 C. for 30-60 minutes, depending on the weight of the briquette, until residue water in the briquette reaches a value of 1.5% at maximum. After drying, the hot briquette is cooled. The cooled briquette is wrapped for protection against humidification until further use or processing.
For the blending of the components into a homogenous state in the mixer, it is useful if the mixture is preheated to a temperature between 25 and 99xc2x0 C.
After drying and before wrapping, it is essential to cool the hot briquette to a temperature between 30 and 50xc2x0 C.
The main advantage of this invention is that slag can be used as a liquefying agent in metallurgy, e.g. in the manufacture of iron and steel to lower the viscosity of the melt, which considerably reduces costs. Thus a waste product generated by the manufacture of aluminium is utilised, as this waste product stored in heaps decomposes under the influence of various climactic conditions, simultaneously giving off foul-smelling or harmful substances such as ammonia, methane, hydrogen and phosphines. This represents another benefit of the invention, namely the improvement of the environment, including the working environment, and the release of utility areas previously used for slag heaps. Thus it is possible to process the portion of scum that is difficult to process into an environmentally friendly product.
In order to obtain briquettes with sufficient strength under a pressure of at least 20 MPa, it is proposed that the composition of briquettes include a polymeric bonding agent, namely polyvinyl acetate. Briquettes for the liquefaction of metallurgical slag may have varying degrees of strength and stability during storage, depending on the choice of polymeric substance and its composition. Briquettes of cylindrical shape weighing 50 to 250 g have the greatest strength. To guarantee equal distribution of the bonding particles of the polymeric substance between the particles of slag, the briquettes contain water.
According to its physical properties, slag is a consolidated substance. The specific weight of slag before processing into briquette form is approx. 1,100 kg.mxe2x88x923. In order to obtain briquettes of sufficient strength, it is essential that their specific weight amount to between 1,500 and 5,000 kg.mxe2x88x923.
The proposed invention uses polyvinyl acetate, a very simple polymer that is cheap to manufacture: 
The polymer has one radical, namely the acetate group OCOCH3.
When this polymer decomposes, it releases substances that are not harmful to living organisms. These substances include water, carbon dioxide and acetic acid. In essence it is a polymeric, coarsely disperse, non-plasticized, viscous liquid, slightly yellow white in colour, with particles 1-3 micrometers in sized, devoid of granules or alien mechanical intrusions. The weight proportion of dry residue is at least 51%. The weight proportion of remanent monomer is up to 0.5%. The usual viscosity, according to the standardised measuring-vessel measurement, is 11-40, expressed as a Saybolt number. Adhesive power is at least 450 N/.mxe2x88x921.
So that the briquettes according to this invention may be used for the liquefaction of metallurgical slag, they must have not only sufficient strength and stability in storage, but also a minimal cost. If the quantity of the polymeric ingredient in the composition of the briquettes is increased, the briquettes"" strength and resistance t decomposition is increased, but the cost of manufacture rises sharply to 10-20 times the amount. For this reason, to preserve a minimal cost of manufacture whilst obtaining the necessary strength and resistance of the briquettes, the polymeric substance may be partly replaced by hydrophobic ingredients. The addition of at least one hydrophobic ingredient to the mixture for briquette manufacture, in a quantity of 0.5-2% by weight, acts in a water-resistant way and significantly reduces the absorption capacity of the briquette whilst considerably lengthening the storage period. For the hydrophobic ingredient, unsaturated acids such as palmitic acid, stearic acid and oleic acid are used. The acid used most often is oleic acid, described by the chemical formula C17 H33 COOH, which exists in two crystalline modifications, alpha and beta, with melting temperatures of 13.4 and 16.3xc2x0 C. respectively. At room temperature this acid has a specific weight of 0.089 g.cmxe2x88x923. Both of these acids may be used as pure chemicals or in mixtures of 0 to 100% in mutual combination. These acids reduce the hydroscopic properties of the mixture and the subsequent absorption of atmospheric humidity. They may also be used as lubricants in pressing machines.
For an equal distribution of the bonding particles of the polymeric substances and the hydrophobic ingredient among the particles of slag, as well as for the reduction of the internal and external abrasive forces arising as a consequence of pressing, water is added to the briquettes. The water may be either service water or tap water.
So that the briquettes from secondary aluminium processing may be used for the liquefaction of metallurgical slag, they must have sufficient strength and resistance to decomposition. The manufacture of briquettes from secondary aluminium processing using ordinary humidifying technology is not feasible, as the briquettes thus obtained decompose in 20-40 minutes while drying in the air. This occurs because some of the components of the slag, e.g. the aluminium nitrides AlxNy and Al aluminium, enter into a chemical reaction with the water, generating, heat and gases such as ammonia and hydrogen. The gases thus released disrupt the bonding between the particles and the briquettes disintegrate.
The components of the briquette mixture are mixed into a homogenous state in a mixer in a time of 10-60 seconds. The mixing time of 10 seconds is the minimum time necessary for the mixture to be fully moistened, whilst 60 seconds is the maximum mixing time in order to avoid speeding up the heightened decomposition of the slag. At first, the bonding ingredients and the water are mixed in a special mixer of e.g. circular type. In this type of mixer, the polymeric dispersion is subsequently mixed with the slag from secondary aluminium processing. The mixture is deemed homogenous when the homogenised briquette mixture compresses slightly and its component ingredients do not separate.
The pressing of the briquettes is carried out at a specific press power of 110 to 130 MPa, which guarantees the compression strength of the briquettes and ensures that the bonding agent and any hydrophobic ingredient is squeezed out to the surface of the slag grains.
To dry the briquettes, a temperature of 120-200xc2x0 C. is recommended, for a time of 30-60 minutes depending on the weight of the briquette, so that the water necessary for soaking the surface of the slag can be removed in order to avoid unwanted decomposition of the slag. Thus the temperatures for polymerising and drying the briquettes are sufficiently low, reducing the cost of the processing. Drying can be done in a drying apparatus of rotational type.
After drying, the hot briquettes are usually easily cooled. If this does not take place, there is a risk that they will re-absorb humidity and disintegrate. The briquettes can be artificially cooled to 30-50xc2x0 C., according to the type of packaging used thereafter. Cooling may be carried out naturally or artificially in a cooling conveyer.
The cooled briquettes are wrapped up for protection against humidity by, for example, atmospheric precipitation and excess humidity, until further use or processing.
The proposed method allows part of the scums that are difficult to process to be processed into a product which is acceptable with regard to protection of the environment and which saves primary raw material resources, especially fluorite, quartz sands, aluminium oxide or bauxite.